1. Field of the Invention
The present invention relates to a signal transmission cable and a multi-wire cable having excellent electric characteristics, mechanical characteristics, and terminal workability.
2. Related Art
As a cable used for signal transmission between a main body and a liquid-crystal display in small-sized electronic devices such as notebook-sized personal computer, portable telephone and the like, a superfine coaxial cable has been used, since predetermined electric characteristics for EMI (Electromagnetic Interference) and SKEW (a clock skew) are required. For example, Japanese Patent Laid-Open No. 2002-352640 (JP-A-2002-352640) discloses an example of the super fine coaxial cables.
As shown in FIG. 9, a superfine coaxial cable 91 comprises an inner conductor 92, an insulating layer 93 provided at an outer periphery of the inner conductor 92, an outer conductor 94 provided at an outer periphery of the insulating layer 93, and a sheath 95 provided at an outer periphery of the outer conductor 94.
A method of signal transmission between the main body and the liquid-crystal display in the notebook-sized personal computer is shifted from a parallel transmission method to a serial transmission method. Since strict electric characteristics are required in a cable for the serial transmission compared with those of the superfine coaxial cable, two-wire (core) coaxial cables, four-wire stranded common shield cables are used. For example, Japanese Patent Laid-Open No. 2003-22718 (JP-A-2003-22718) discloses an example of the two-wire coaxial cables. Japanese Patent Laid-Open No. 2003-132743 (JP-A-2003-132743) and a Japanese publication No. 9-511359 of translation of WO96/24143 (JP-T-9-511359) disclose examples of the four-wire stranded common shield cables.
As shown in FIG. 10, a two-wire coaxial cable 101 comprises two inner insulated wires 104 each having an inner conductor 102 and an insulating layer 103 provided at an outer periphery of the inner conductor 102, an outer conductor 105 provided at outer periphery of the inner insulated wires 104, and a sheath 106 provided at an outer periphery of the outer conductor 105.
As shown in FIG. 11, a four-wire stranded common shield cable 111 comprises four inner insulated wires 114 each having an inner conductor 112 and an insulating layer 113 provided at an outer periphery of the inner conductor 112, the inner insulated wires 114 being stranded around an outer periphery of a filler 115, an outer conductor 116 provided at outer periphery of the inner insulated wires 114, and a sheath 117 provided at an outer periphery of the outer conductor 116.
The number of the portable telephones using the two-wire coaxial cable is increased. As for the two-wire coaxial cable, bending resistance property (flexibility) and twisting resistance property are highly requested, and the number of internal antennas for increasing various receiving functions is increased, so that high EMI characteristics are requested.
These cables are used in a manner that a plurality of cables are arranged in parallel, and a terminal portion of the cable is made flat and connected onto a board at a connector side. This terminal work (processing) is conducted by laser beam machining with use of YAG laser, however, it is necessary for preventing the inner conductor from being damaged by the irradiation of the laser beam.
As a technique of directly cutting the outer conductor by the laser beam machining without damaging the inner conductor, Japanese Patent Laid-Open No. 2005-251522 (JP-A-2005-251522) proposes a technique of coloring a fluorine resin which is a main material of the insulating layer covering the inner conductor into “pale black” by doping 0.025 wt % to 0.14 wt % of carbon black.
As a technique of composing a cable in which the outer conductor can be cut without damaging the inner conductor, Japanese Patent Laid-Open No. 2004-192815 (JP-A-2004-192815) proposes a technique of doping powdery additives to the resin which is the main material of the insulating layer covering the inner conductor. The powdery additive is made by mixing an additive with white color or metallic color which easily provides a total reflection of the laser beam, an additive with black color which easily absorbs the laser beam, and a colorant made of a metallic oxide.
These cables are also called as differential signal transmission cable, since a differential signal is transmitted between the main body and the liquid-crystal display.
However, there are following disadvantages in structure of the conventional cable.
Since the two-wire coaxial cable 101 shown in FIG. 10 has an elliptical cross section, a symmetrical property at 360° around the two-wire coaxial cable 101 is not good, so that it is not suitable for the application of twisting the cable multi-axially, for example, in the portable telephone.
In the four-wire stranded common shield cable 111 shown in FIG. 11, a pitch between the inner insulated wires 114 is easily varied when the cable is bent or twisted, so that the electric characteristics are instable, and variation in the electric characteristics is large. In addition, when the terminal processing for arranging the inner insulated wires 114 in a pitch of 0.3 mm to 0.5 mm, there are many failures in that tip portions of strands (bare wires) wound around the inner insulated wires 114, which constitute the outer conductor 116, stick into the insulating layer 113 of the inner insulated wire 114, so that the strands are short-circuited with the inner conductors 112.
In the four-wire stranded common shield cable 111, when wrapping a copper-evaporated PET tape over an outer periphery of stranded four inner insulated wires 114 to provide the outer conductor 116, the cable has a high hardness and the mechanical characteristics such as the flexibility, the twist-resistance property are not good while the electric characteristics are stable. Therefore, the four-wire stranded common shield is not suitable for the application of twisting the cable multi-axially, for example, in the portable telephone.
Further, in the conventional cable, there is a problem of the terminal processing, in that it is difficult to conduct the laser beam machining with the use of YAG laser. Namely, the laser beam transmits through gaps between the strands in the outer conductor, thereby damaging the insulating layer of the inner insulated wires as well as the inner conductor.
As for the superfine coaxial cable 91 shown in FIG. 9 (disclosed by JP-A-2002-352640), in the case that a thickness of the insulating layer 93 is relatively large, e.g. 60 μm, the insulation characteristic is high, namely, an acceptance rate of the insulation resistance test reaches 100%. On the other hand, in the case that the thickness of the insulating layer 93 is relatively small, e.g. 40 μm, 50 μm, namely, less than 60 μm, the insulation characteristic is low. Therefore, when the carbon black is added to the insulating layer 93, it is necessary to increase the thickness of the insulating layer 93 to maintain the high insulation characteristic. As a result, the diameter of the cable is increased.
Still further, as for the cable comprising a plurality of the inner insulated wires as shown FIGS. 10 and 11, when the color of the insulating layer is totally colored by black, it is difficult to discriminate the inner insulated wires by visual inspection. On the other hand, when a color other than black is used for the insulating layer, for example, when only one of the insulating layers is colored by black and the other insulating layers are colored by different colors that are distinguishable from black, the insulating layer and the inner conductor of the inner insulated wire having the insulating layer colored by the color other than black will be damaged when the outer conductor is cut by the laser beam machining. The reason why the insulating layer and the inner conductor of the inner insulated wire are not damaged when the color of the insulating layer is black is not perfectly elucidated yet. However, the Inventors contemplated that the black insulating layer is doped with the carbon black as a color pigment and an optical transmission rate of the carbon black is remarkably small compared with other color pigments, so that the carbon black has an effect of suppressing the damage of the inner conductor by intercepting the light. In addition, the Inventors contemplated that although the carbon black generates the heat by radiation of the laser beam since the carbon black has a characteristic of absorbing more light than the other color pigments, a temperature of the generated heat in the carbon black is lower than a softening temperature of the fluorine resin (about 302° C.), so that the fluorine resin will not be molten by the generated heat.
The cable disclosed by JP-A-2004-192815 is not practical, since kind of additives to be added to the resin which is the main material of the insulating layer, combination thereof, and doping amount thereof are not disclosed.